Field of the Invention
The present invention generally relates to a welding device and, more particularly, to a spot welding device for nuclear fuel skeleton which welds guide tubes for control rods and a instrumentation tube for measuring the state inside a nuclear reactor to a spacer grid supporting fuel rods in the nuclear reactor.
Description of the Related Art
A nuclear fuel skeleton is a frame for keeping fuel rods and for checking the reaction state of nuclear fuel in a nuclear reactor of a nuclear power plant. The skeleton is composed of spacer grids having a plurality of cells for fuel rods, guide tubes for control rods to be put into holes formed in the spacer grids arranged with a predetermined distance therebetween, and a instrumentation tube for measuring the state inside a nuclear reactor. The spacer grid is an important part of a nuclear fuel that fixes and supports fuel rods with uranium therein with predetermined force at predetermined distances and improves heat transfer from a nuclear reactor to a coolant.
A fuel assembly is formed by putting fuel rods into the spacer grid cells of the skeleton and nozzles to both ends of the skeleton, and although it depends on the particular models of power plants, approximately one hundred seventy-seven fuel assemblies are used for about four years in nuclear reactors under Koran standard nuclear fuel regulations.
As shown by the Chernobyl nuclear accident in the Soviet Union in 1986, a nuclear power plant requires very strict safety protocols. In particular, the nuclear fuel skeleton requires durability against extreme conditions for a long period of time in a nuclear reactor, so it is very important to perform many examinations in the process of manufacturing the nuclear fuel skeleton in order to achieve a high quality product.
The spacer grids, guide tubes, and instrumentation tube of a skeleton are formed by bulging or welding. TIG welding has been used as the welding method in the related art. All the processes of TIG welding are manually carried out in a closed chamber filled with an argon gas due to the properties of the Zirconium Alloy that is a material for spacer grids and tubes (guide tubes and instrumentation tube). This is because If TIG welding is performed in a standby state, the metal at the welded portion combined with oxygen is broken due to the metallic properties of Zirconium Alloy, so bonding is impossible.
Another reason for the requirement of manual welding is that it is difficult to set a welding angle due to narrow spaces between spacer grids, guide tubes, and a instrumentation tube, and inter-electrode contact is created at other portions of a skeleton during the process. Further, because there is a need for welding at over forty points within each grid when assembling one skeleton, the welding process is difficult.
Additionally, another reason for the requirement of manual welding of spacer grids, guide tubes, and a instrumentation tube, is that a skeleton with even a slight defect cannot be reused; thus the defect causes an economic loss.
Accordingly, for those reasons, it has been required to automate the process of assembling a skeleton that necessarily has low productivity and is expensive.
To address the above, the applicant(s) has applied for a patent titled “Robot spot welding machine for nuclear fuel skeleton and spot welding method using that”, which has been disclosed in Korean Patent No. 10-0526721.
The patent is shown in FIGS. 1 to 3. The registered patent will be simply introduced and the necessity of improving the structure of welding gun will be described hereafter mainly about the matters not solved by the registered patent.
First, FIG. 1 is a front view showing the entire configuration of a welding device of the registered patent.
The robot spot welding device of the registered patent largely includes: a welding bench 200 with fixing frames 220 arranged at predetermined distances from each other to correspond to the gaps between spacer grids 110 constituting a skeleton 100; a robot 300 having a welding gun 350 and arranged in parallel with the welding bench, that is, on the rear side of the bench in the figures; electrodes inserted in guide tubes 120 and a instrumentation tube 130 to weld the spacer grids 110 and the guide tubes 120 to each other and the spacer grids and the instrumentation tube 130 to each other; and an loading table 500 aligned with the welding bench 200 to put the tubes 120 and 130 horizontally into holes 117a, 117b, 117c, 117d, and 119 formed in the spacer grids 110 to align with the height of the holes these devices.
The side with the welding bench 200 where the loading table 500 is positioned is defined as an ‘upper part’ and the opposite side is defined as a ‘lower part’ hereafter for the convenience of description.
As described above, the types of nuclear fuel skeletons depend on the types of power plants, and manufacturers. FIGS. 2A and 2B shows a spacer grid 111 of a guardian type assembly for KSNP that has been used in the past of the grids for the skeleton. Though not used now, the arrangement of guide tubes and a instrumentation tube in the spacer grid is the same as that in the spacer grid of a PLUS7 assembly for KSNP shown in FIG. 10, which is generally used at present.
As can be seen from the front view of FIG. 2A, the grid 111 is formed by laser welding on plates made of Zirconium Alloy and arranged across each other. Springs 115a for fixing fuel rods F are formed in cells 115 formed by the grid plates.
Further, as can be seen from the side view of FIG. 2B, two dimples 115b are formed on the wall of each of the cells 115 for firmly fixing the fuel rods F, that is, total eight dimples are formed.
As holes for inserting tubes in FIG. 2A, a hole 119 for a instrumentation tube 130 and holes 117a, 117b, 117c, and 117d for guide tubes 120a, 120b, 120c, and 120d are formed.
In FIG. 2B, the welding points of the tubes 120 and 130 and the grid 111 are indicated by ‘W’, and according to TIG welding of the related art, welding is performed not inside the cells, but at the outsides where both ends of the cells and the tubes are in contact with each other.
Such a configuration allows for automation of welding of a spacer grid, which has been performed manually in the past.
However, as can be seen from FIG. 3, it is impossible to weld the instrumentation tube 130 with the welding gun 350 selected in the registered patent due to interference with the guide tubes, particularly, the guide tube 120d in the entrance of the welding gun, after welding the upper guide tubes 120c and 120d. 
Accordingly, in order to weld the instrumentation tube 130 and all the guide tubes 120a, 120b, 120c, and 120d with the welding gun 350, it is required to follow the welding sequence of the lower guide tubes 120a and 120b, the instrumentation tube 130, and the upper guide tubes 120c and 120d. Further, welding or repairing of the instrumentation tube 130 or the lower guide tubes 120a and 120b for several purposes including maintenace, after the upper guide tubes 120c and 120d are welded, has to be performed manually. Therefore, there is a need for developing a welding gun that can weld the instrumentation tube 130 or the lower guide tubes 120a and 120b, even if the upper guide tubes 120c and 120d are welded, regardless of the welding sequence.
The guardian type assembly for KSNP is not used any more, and the PLUS7 assembly for KSNP shown in FIG. 10 is used now. Sleeves that can be combined with guide tubes and a instrumentation tube are inserted in advance at the positions for the guide tubes and the instrumentation tube in the spacer grid of the PLUS7 assembly for KSNP, as can be seen from FIG. 10, so the electrode of a welding gun is not directed to the inside of the spacer grid in welding, but welding is performed at the positions W where the sleeves and the guide tubes or the sleeves and the instrumentation tube overlap each other. Therefore, it is also required to change the direction of the electrode of the welding gun according to the related art shown in FIG. 9.
The foregoing is intended merely to aid in the understanding of the background of the present invention, and is not intended to mean that the present invention falls within the purview of the related art that is already known to those skilled in the art.